The present invention is directed generally to optical systems. More particularly, the invention relates to optical systems including optical amplifiers and methods for use therein.
The continued growth in traditional communications systems and the emergence of the Internet as a means for accessing and communicating information has accelerated demand for high capacity communications networks. Telecommunications service providers, in particular, have looked to wavelength division multiplexed (“WDM”) transmission systems to increase the capacity of their optical fiber networks to meet the increasing demand.
In WDM transmission systems, distinct wavelength ranges that are useful for transmission through a transmission medium are allocated to carry separate information streams simultaneously within the medium. Analogously, distinct frequency ranges can be allocated to carry separate information streams in frequency division multiplexed (“FDM”) systems. The wavelength/frequency ranges of WDM, FDM, and other systems carrying multiple information streams are often referred to signal wavelengths/frequencies, or signal channels. The ranges are characterized by a center wavelength/frequency, which is typically the mid-point of the wavelength/frequency range. The ranges also may be characterized in other manners, such as the wavelength/frequency of maximum power or a relative to reference wavelength/frequency.
In WDM systems, signal channels are transmitted using electromagnetic waves within the distinct wavelength ranges in the optical spectrum, typically in the infrared wavelength range. Each signal channel can be used to carry a single information stream or multiple information streams that are electrically or optically time division multiplexed (“TDM”) together into a TDM information stream.
The pluralities of information carrying wavelengths are combined into a multiple channel, “WDM”, optical signal that is transmitted in a single waveguide. In this manner, WDM and other multiple channel systems can increase the transmission capacity of space division multiplexed (“SDM”), i.e., single channel, optical systems by a factor equal to the number of channels in the multiple channel system.
The development of optical amplifiers capable of simultaneously amplifying multiple optical signals greatly reduced the cost of optical systems, and WDM systems in particular. This capabilityessentially eliminated the need for expensive electronic repeater equipment to separate and repeat each signal electrically merely to overcome signal attenuation.
While the development of optical amplifiers has greatly reduced the equipment costs and increased reliability associated with amplifiers in optical systems, there remain operational concerns. A failure in an optical amplifier could prevent optical signals from passing through the amplifier and the system. As such, various techniques have been developed to mitigate the impact of a failure in an optical amplifier. For example, redundant pump lasers, or pumps, have been used to provide optical, or “pump”, power to the amplifier, so that failure of a pump laser would not cause a failure of the amplifier. However, the additional cost associated with redundant pumps often does not provide a cost effective solution to this problem. Alternatively, pump power from a multiple pump lasers can been combined and shared among two or more amplifiers to minimize or eliminate the cost associated with pump laser redundancy. However, the sharing of pump lasers requires that the amplifiers be operated in tandem, which significantly constrains the operation of the individual amplifiers.
In addition, the amount of optical amplification required in a system depends upon the system design. For example, various system designs require chromatic dispersion compensation to reduce the detrimental impact on a signal of chromatic dispersion, which inherently results from multi-wavelength signal propagation through a transmission fiber. Various techniques have been developed to compensate for chromatic dispersion in the fiber. The most commonly used technique involves the use of dispersion compensating fiber (“DCF”), which has been designed to have dispersion characteristics opposite that of a transmission fiber or a second transmission fiber is used that has different dispersion characteristics than the transmission fiber used in a span and/or system. Some DCF is designed to have significantly more dispersion, e.g., 10×, than the transmission fiber, such that shorter lengths of DCF are required to compensate for the dispersion in the transmission fiber. The design of DCF typically results in a fiber that has a substantially smaller core size, i.e., ½ the diameter of standard single mode fiber.
A problem with DCF is that the DCF can have a significant amount of loss associated with it, which can be, for example, as high as 10 dB or more. To address this problem, DCF is typically deployed proximate, and in combination with, one or more collocated optical amplifiers. However, it is not generally desirable to place DCF proximate the input of an optical amplifier. This is because the additional loss will decrease the input signal power to the optical amplifier, thereby degrading the optical signal to noise ratio (“OSNR”) and effective noise figure of the optical amplifier. Conversely, it is not desirable to place DCF proximate the output of an optical amplifier. This is due to the small core size of the DCF, which can dramatically increase the non-linear signal interactions and degrade the optical signal to noise ratio (“OSNR”) and effective noise figure of the optical amplifier. As such, DCF is typically placed proximate, i.e., between, two amplifiers, amplification sections, or amplifier stages. In this location, the signal power entering is higher than at the input to the first amplification section and lower than the output of the second amplification section. However, even in this location, the signal channel powers and non-linear interactions are high and the optical amplifiers and system must be designed to accommodate the additional loss and performance degradation resulting from the DCF. Some efforts have been made to ameliorate the loss by providing Raman gain in the DCF between the amplifier stages. However, these efforts, while reducing the effective loss through the DCF can actually increase the signal channel power and non-linear interactions within the DCF.
There is a continuing interest in the development of higher performance, lower cost communication systems. As such, there is a continuing need for improved optical systems, amplifiers and amplification methods.